Preconditioning with endoplasmic reticulum stress mitigates retinal endothelial inflammation via activation of X-box binding protein 1

The Unfolded Protein Response in Homeostasis and Modulation of Mammalian Immune Cells

The endoplasmic reticulum (ER) plays important roles in eukaryotic protein folding and lipid biosynthesis. Several exogenous and endogenous cellular sources of stress can perturb ER homeostasis leading to the accumulation of unfolded proteins in the lumen. Unfolded protein accumulation triggers a signal-transduction cascade known as the unfolded protein response (UPR), an adaptive mechanism which aims to protect cells from protein aggregates and to restore ER functions. Further to this protective mechanism, in immune cells, UPR molecular effectors have been shown to participate in a wide range of biological processes such as cell differentiation, survival and immunoglobulin and cytokine production. Recent findings also highlight the involvement of the UPR machinery in the maturational program and antigen presentation capacities of dendritic cells. UPR is therefore a key element in immune system homeostasis with direct implications on both adaptive and innate immune responses. The present review summarizes the knowledge on the emerging roles of UPR signaling cascades in mammalian immune cells as well as the consequences of their dysregulation in relation to the pathogenesis of several diseases.

Phenotypic assays identify azoramide as a small-molecule modulator of the unfolded protein response with antidiabetic activity

The endoplasmic reticulum (ER) plays a critical role in protein, lipid, and glucose metabolism as well as cellular calcium signaling and homeostasis. Perturbation of ER function and chronic ER stress are associated with many pathologies ranging from diabetes and neurodegenerative diseases to cancer and inflammation. Although ER targeting shows therapeutic promise in preclinical models of obesity and other pathologies, the available chemical entities generally lack the specificity and other pharmacological properties required for effective clinical translation. To overcome these challenges and identify new potential therapeutic candidates, we first designed and chemically and genetically validated two high-throughput functional screening systems that independently measure the free chaperone content and protein-folding capacity of the ER. With these quantitative platforms, we characterized a small-molecule compound, azoramide, that improves ER protein-folding ability and activates ER chaperone capacity to protect cells against ER stress in multiple systems. This compound also exhibited potent antidiabetic efficacy in two independent mouse models of obesity by improving insulin sensitivity and pancreatic β cell function. Together, these results demonstrate the utility of this functional, phenotypic assay platform for ER-targeted drug discovery and provide proof of principle for the notion that specific ER modulators can be potential drug candidates for type 2 diabetes.

Erp29 Attenuates Cigarette Smoke Extract-Induced Endoplasmic Reticulum Stress and Mitigates Tight Junction Damage in Retinal Pigment Epithelial Cells

PURPOSE

Endoplasmic reticulum protein 29 (ERp29) is a novel chaperone that was recently found decreased in human retinas with AMD. Herein, we examined the effect of ERp29 on cigarette smoke-induced RPE apoptosis and tight junction disruption.

METHODS

Cultured human RPE (HRPE) cells (ARPE-19) or mouse RPE eyecup explants were exposed to cigarette smoke extract (CSE) for short (up to 24 hours) or long (up to 3 weeks) periods. Expression of ERp29 was up- and downregulated by adenovirus and siRNA, respectively. Endoplasmic reticulum stress markers, apoptosis, and cell death, the expression and distribution of tight junction protein ZO-1, transepithelial electrical resistance (TEER), and F-actin expression were examined.

RESULTS

Endoplasmic reticulum protein 29 was significantly increased by short-term exposure to CSE in ARPE-19 cells or eyecup explants but was reduced after 3-week exposure. Overexpression of ERp29 increased the levels of GRP78, p58(IPK), and Nrf-2, while reducing p-eIF2α and C/EBP homologous protein (CHOP), and protected RPE cells from CSE-induced apoptosis. In contrast, knockdown of ERp29 decreased the levels of p58(IPK) and Nrf2, but increased p-eIF2α and CHOP and exacerbated CSE-triggered cell death. In addition, overexpression of ERp29 attenuated CSE-induced reduction in ZO-1 and enhanced the RPE barrier function, as measured by TEER. Knockdown of ERp29 decreased the level of ZO-1 protein. These effects were associated with changes in the expression of cytoskeleton F-actin.

CONCLUSIONS

Endoplasmic reticulum protein 29 attenuates CSE-induced ER stress and enhances cell viability and barrier integrity of RPE cells, and therefore may act as a protective mechanism for RPE survival and activity.

ARMADA: Using motif activity dynamics to infer gene regulatory networks from gene expression data

Analysis of gene expression data remains one of the most promising avenues toward reconstructing genome-wide gene regulatory networks. However, the large dimensionality of the problem prohibits the fitting of explicit dynamical models of gene regulatory networks, whereas machine learning methods for dimensionality reduction such as clustering or principal component analysis typically fail to provide mechanistic interpretations of the reduced descriptions. To address this, we recently developed a general methodology called motif activity response analysis (MARA) that, by modeling gene expression patterns in terms of the activities of concrete regulators, accomplishes dramatic dimensionality reduction while retaining mechanistic biological interpretations of its predictions (Balwierz, 2014). Here we extend MARA by presenting ARMADA, which models the activity dynamics of regulators across a time course, and infers the causal interactions between the regulators that drive the dynamics of their activities across time. We have implemented ARMADA as part of our ISMARA webserver, ismara.unibas.ch, allowing any researcher to automatically apply it to any gene expression time course. To illustrate the method, we apply ARMADA to a time course of human umbilical vein endothelial cells treated with TNF. Remarkably, ARMADA is able to reproduce the complex observed motif activity dynamics using a relatively small set of interactions between the key regulators in this system. In addition, we show that ARMADA successfully infers many of the key regulatory interactions known to drive this inflammatory response and discuss several novel interactions that ARMADA predicts. In combination with ISMARA, ARMADA provides a powerful approach to generating plausible hypotheses for the key interactions between regulators that control gene expression in any system for which time course measurements are available.

Endoplasmic Reticulum Stress in the Diabetic Kidney, the Good, the Bad and the Ugly

Diabetic kidney disease is the leading worldwide cause of end stage kidney disease and a growing public health challenge. The diabetic kidney is exposed to many environmental stressors and each cell type has developed intricate signaling systems designed to restore optimal cellular function. The unfolded protein response (UPR) is a homeostatic pathway that regulates endoplasmic reticulum (ER) membrane structure and secretory function. Studies suggest that the UPR is activated in the diabetic kidney to restore normal ER function and viability. However, when the cell is continuously stressed in an environment that lies outside of its normal physiological range, then the UPR is known as the ER stress response. The UPR reduces protein synthesis, augments the ER folding capacity and downregulates mRNA expression of genes by multiple pathways. Aberrant activation of ER stress can also induce inflammation and cellular apoptosis, and modify signaling of protective processes such as autophagy and mTORC activation. The following review will discuss our current understanding of ER stress in the diabetic kidney and explore novel means of modulating ER stress and its interacting signaling cascades with the overall goal of identifying therapeutic strategies that will improve outcomes in diabetic nephropathy.

CHOP mediates XBP1S-induced renal mesangial cell necrosis following high glucose treatment

High glucose (HG)-induced apoptosis in mesangial cells (MCs) is a critical determinant during the pathogenesis of diabetic nephropathy. The signaling cascade inducing MCs apoptosis by HG involves overproduction of reactive oxygen species. Our previous studies have demonstrated that HG-induced oxidative stress is mediated by suppression of spliced/active X-box binding protein 1 (XBP1S), suggesting the importance of XBP1S in HG-induced MCs apoptosis. CHOP, an endoplasmic reticulum stress-associated proapoptotic signal, is involved in downstream of XBP1S. In the present study, we explored the effect of XBP1S in modulating HG-induced apoptosis in renal MCs and then identified the role of CHOP in these processes. Apoptosis and necrosis were quantified by flow cytometry; protein levels of XBP1S, caspase3, Bax, Bcl2, BNIP3, and CHOP were analyzed by Western blotting. The cellular localization of XBP1S was determined by immunofluorescence histochemistry. The binding of XBP1 to CHOP promoter was determined by chromatin immunoprecipitation assays. In addition, adenoviruses harboring XBP1S gene (Ad-XBP1S) were used to overexpress XBP1S, whereas the knockdown of CHOP was achieved by small interference RNA. HG suppressed nuclear distribution of XBP1S and induced apoptosis and necrosis in MCs. Ad-XBP1S infection enhanced the nuclear translocation of XBP1S and reduced MCs apoptosis and necrosis. XBP1S bound to the promoter region of CHOP and upregulated CHOP expression. Conversely, CHOP expression was reduced upon HG exposure and knockdown of CHOP increased necrosis but not apoptosis in MCs. These results suggest that XBP1S protected MCs from HG-induced apoptosis and necrosis, and CHOP participates in XBP1S-regulated necrosis but not apoptosis.

Involvement of the IRE1α-XBP1 pathway and XBP1s-dependent transcriptional reprogramming in metabolic diseases

The X-box binding protein 1 (XBP1) is not only an important component of the unfolded protein response (UPR), but also an important nuclear transcription factor. Upon endoplasmic reticulum stress, XBP1 is spliced by inositol-requiring enzyme 1 (IRE1), thereby generating functional spliced XBP1 (XBP1s). XBP1s functions by translocating into the nucleus to initiate transcriptional programs that regulate a subset of UPR- and non-UPR-associated genes involved in the pathophysiological processes of various diseases. Recent reports have implicated XBP1 in metabolic diseases. This review summarizes the effects of XBP1-mediated regulation on lipid metabolism, glucose metabolism, obesity, and atherosclerosis. Additionally, for the first time, we present XBP1s-dependent transcriptional reprogramming in metabolic diseases under different conditions, including pathology and physiology. Understanding the function of XBP1 in metabolic diseases may provide a basic knowledge for the development of novel therapeutic targets for ameliorating these diseases.

Activation of the UPR protects against cigarette smoke-induced RPE apoptosis through up-regulation of Nrf2

Recent studies have revealed a role of endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) in the regulation of RPE cell activity and survival. Herein, we examined the mechanisms by which the UPR modulates apoptotic signaling in human RPE cells challenged with cigarette smoking extract (CSE). Our results show that CSE exposure induced a dose- and time-dependent increase in ER stress markers, enhanced reactive oxygen species (ROS), mitochondrial fragmentation, and apoptosis of RPE cells. These changes were prevented by the anti-oxidant NAC or chemical chaperone TMAO, suggesting a close interaction between oxidative and ER stress in CSE-induced apoptosis. To decipher the role of the UPR, overexpression or down-regulation of XBP1 and CHOP genes was manipulated by adenovirus or siRNA. Overexpressing XBP1 protected against CSE-induced apoptosis by reducing CHOP, p-p38, and caspase-3 activation. In contrast, XBP1 knockdown sensitized the cells to CSE-induced apoptosis, which is likely through a CHOP-independent pathway. Surprisingly, knockdown of CHOP reduced p-eIF2α and Nrf2 resulting in a marked increase in caspase-3 activation and apoptosis. Furthermore, Nrf2 inhibition increased ER stress and exacerbated cell apoptosis, while Nrf2 overexpression reduced CHOP and protected RPE cells. Our data suggest that although CHOP may function as a pro-apoptotic gene during ER stress, it is also required for Nrf2 up-regulation and RPE cell survival. In addition, enhancing Nrf2 and XBP1 activity may help reduce oxidative and ER stress and protect RPE cells from cigarette smoke-induced damage.

Identification of p58IPK as a novel neuroprotective factor for retinal neurons

PURPOSE

Endoplasmic reticulum (ER)-resident chaperone protein p58(IPK) plays a vital role in regulation of protein folding and biosynthesis. The goal of this study was to examine the role of p58(IPK) in retinal neuronal cells under normal and stressed conditions.

METHODS

Retinal expression of p58(IPK), retinal morphology, apoptosis, ER stress, and apoptotic gene expression were examined in p58(IPK) knockout (KO) and/or wild-type (WT) mice with or without intravitreal injection of N-methyl-D-aspartic acid (NMDA). In in vitro experiments, differentiated R28 retinal neuronal cells transduced with adenovirus encoding p58(IPK) (Ad-p58(IPK)) or control virus (Ad-LacZ) were exposed to tunicamycin (TM) or hydrogen peroxide (H2O2). Levels of ER stress, apoptosis, and cell survival were evaluated.

RESULTS

Chaperone protein p58(IPK) is expressed predominantly in retinal ganglion cells (RGC), inner retinal neurons, and the photoreceptor inner segments. Mice lacking p58(IPK) exhibited increased CHOP expression and loss of RGCs with aging (8-10 months). Intravitreal injection of NMDA induced retinal ER stress and increased p58(IPK) expression in WT mice; this resulted in greater ER stress and enhanced RGC apoptosis in p58(IPK) KO mice. In cultured R28 cells, overexpression of p58(IPK) significantly reduced eIF2α phosphorylation, decreased CHOP expression, and alleviated the activation of caspase-3 and PARP. Overexpression of p58(IPK) also protected against oxidative and ER stress-induced cell apoptosis. Furthermore, p58(IPK) downregulated the proapoptotic gene Bax and upregulated the antiapoptotic gene Bcl-2 expression in stressed R28 cells.

CONCLUSIONS

Our study has demonstrated a protective role of p58(IPK) in retinal neurons, which may act in part through a mechanism involving modulation of ER homeostasis and apoptosis, particularly under conditions of cellular stresses.

The unfolded protein response in retinal vascular diseases: implications and therapeutic potential beyond protein folding

Angiogenesis is a complex, step-wise process of new vessel formation that is involved in both normal embryonic development as well as postnatal pathological processes, such as cancer, cardiovascular disease, and diabetes. Aberrant blood vessel growth, also known as neovascularization, in the retina and the choroid is a major cause of vision loss in severe eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinopathy of prematurity, and central and branch retinal vein occlusion. Yet, retinal neovascularization is causally and dynamically associated with vasodegeneration, ischemia, and vascular remodeling in retinal tissues. Understanding the mechanisms of retinal neovascularization is an urgent unmet need for developing new treatments for these devastating diseases. Accumulating evidence suggests a vital role for the unfolded protein response (UPR) in regulation of angiogenesis, in part through coordinating the secretion of pro-angiogenic growth factors, such as VEGF, and modulating endothelial cell survival and activity. Herein, we summarize current research in the context of endoplasmic reticulum (ER) stress and UPR signaling in retinal angiogenesis and vascular remodeling, highlighting potential implications of targeting these stress response pathways in the prevention and treatment of retinal vascular diseases that result in visual deficits and blindness.

Emerging functions of the unfolded protein response in immunity

The unfolded protein response (UPR) has traditionally been viewed as an adaptive response triggered by the accumulation of unfolded proteins in the endoplasmic reticulum (ER) and aimed at restoring ER function. The UPR can also be an anticipatory response that is activated well before the disruption of protein homeostasis. UPR signaling intersects at many levels with the innate and adaptive immune responses. In some types of cells of the immune system, such as dendritic cells (DCs) and B cells, particular sensors that detect the UPR seem to be constitutively active in the absence of induction of the traditional UPR gene program and are necessary for antigen presentation and immunoglobulin synthesis. The UPR also influences signaling via Toll-like receptors (TLRs) and activation of the transcription factor NF-κB, and some pathogens subvert the UPR. This Review summarizes these emerging noncanonical functions of the UPR in immunity.

Endoplasmic reticulum stress in immunity

Immune responses occur in the midst of a variety of cellular stresses that can severely perturb endoplasmic reticulum (ER) function. The unfolded protein response is a three-pronged signaling axis dedicated to preserving ER homeostasis. In this review, we highlight many important and emerging functional roles for ER stress in immunity, focusing on how the bidirectional cross talk between immunological processes and basic cell biology leads to pleiotropic signaling outcomes and enhanced sensitivity to inflammatory stimuli. We also discuss how dysregulated ER stress responses can provoke many diseases, including autoimmunity, firmly positioning the unfolded protein response as a major therapeutic target in human disease.

Endoplasmic reticulum stress preconditioning antagonizes low-density lipoprotein-induced inflammation in human mesangial cells through upregulation of XBP1 and suppression of the IRE1α/IKK/NF-κB pathway

Elevated plasma low‑density lipoprotein (LDL) is associated with systemic inflammation, and is an important factor in the pathogenesis of chronic kidney disease. The aim of the present study was to investigate the effects of endoplasmic reticulum (ER) stress preconditioning on LDL‑induced inflammatory responses, in human mesangial cells (HMCs). HMCs were exposed to LDL (200 nm), with or without pretreatment with tunicamycin, an ER stress inducer, and tested for changes to gene expression levels. Small interfering RNA technology was used to knockdown the expression of inositol‑requiring enzyme‑1α (IRE1α) and X‑box‑binding protein‑1 (XBP‑1), in order to determine their effects on LDL‑treated HMCs. LDL treatment resulted in a significant, and time‑dependent, increase in the relative mRNA expression levels of proinflammatory cytokines and CD40, which was coupled with enhanced phosphorylation of IRE1α, IκB kinase (IKK), and nuclear factor (NF)‑κB p65 and p65 nuclear translocation. The LDL‑induced inflammatory responses were significantly reduced in the IRE1α‑depleted HMCs. Furthermore, pretreatment with tunicamycin significantly attenuated the induction of proinflammatory cytokines and CD40, by LDL. Whereas, silencing XBP1 expression significantly restored the production of proinflammatory cytokines, in the LDL‑treated HMCs with ER stress preconditioning. The phosphorylation levels of IRE1α, IKK, and NF‑κB p65 were markedly increased in the XBP1‑depleted HMCs. Conversely, overexpression of XBP1 blocked LDL‑induced inflammation in the HMCs. The results of the present study demonstrate that ER stress preconditioning antagonizes LDL‑induced inflammatory responses in HMCs, which may be mediated through upregulation of XBP1, and subsequent inactivation of the IRE1α/IKK/NF‑κB pathway.

C/EBP homologous protein drives pro-catabolic responses in chondrocytes

Introduction

Excess C/EBP homologous protein (CHOP) expression is one feature of the unfolded protein response (UPR) to endoplasmic reticulum (ER) stress. Here, we focused on CHOP expression and function in chondrocytes.

Methods

We studied human knee osteoarthritis (OA) cartilage, bovine chondrocytes cultured in alginate and subjected to sub-lethal biomechanical injury, and knee chondrocytes of human autopsy donors. We performed siRNA knockdown and transfection.

Results

UPR activation was increased in human knee OA cartilage in situ, and in biomechanically injured cultured chondrocytes in vitro. In normal human chondrocytes, CHOP “gain of function” sensitized chondrocytes to IL-1β induced nitric oxide (NO) and matrix metalloproteinase (MMP)-3 release without inducing these responses by itself. Excess CHOP expression, by itself, induced superoxide production and apoptosis. Conversely, siRNA knockdown of CHOP and the UPR-specific mediator X-box binding protein (XBP1) inhibited NO release by >80% (P <0.0005) in response to IL-1β, and blunted MMP-3 release, whereas there were only minimal effects of the UPR mediator GRP78 on these responses. The anti-inflammatory metabolic “super-regulator” AMP kinase (AMPK) is known to limit UPR activation in vascular muscle cells. Here, CHOP supported the capacity of IL-1β to suppress AMPK activity in chondrocytes. We also observed that inhibition of AMPK activity promoted an increase in chondrocyte CHOP expression. Conversely, pharmacologic activation of AMPK by 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR) blunted chondrocyte CHOP expression in response to biomechanical injury.

Conclusions

Biomechanical injury and IL-1 signaling stimulate UPR activation in chondrocytes. CHOP mediates chondrocyte catabolic and apoptotic responses to IL-1β, and does so partly by inhibiting AMPK activity. Conversely, development of excess CHOP activity is limited by AMPK activity in chondrocytes. Our findings suggest a mechanism for potential chondroprotection by AICAR and other AMPK activators. The work is of translational relevance for OA, since several drugs that activate AMPK are already in the clinic for arthritis (for example, allosteric AMPK activators sodium salicylate and high dose aspirin, and methotrexate, which activates AMPK by generating AICAR).

Preconditioning with endoplasmic reticulum stress ameliorates endothelial cell inflammation

Endoplasmic Reticulum (ER) stress, caused by disturbance in ER homeostasis, has been implicated in several pathological conditions such as ischemic injury, neurodegenerative disorders, metabolic diseases and more recently in inflammatory conditions. Our present study aims at understanding the role of ER stress in endothelial cell (EC) inflammation, a critical event in the pathogenesis of acute lung injury (ALI). We found that preconditioning human pulmonary artery endothelial cells (HPAEC) to ER stress either by depleting ER chaperone and signaling regulator BiP using siRNA, or specifically cleaving (inactivating) BiP using subtilase cytotoxin (SubAB), alleviates EC inflammation. The two approaches adopted to abrogate BiP function induced ATF4 protein expression and the phosphorylation of eIF2α, both markers of ER stress, which in turn resulted in blunting the activation of NF-κB, and restoring endothelial barrier integrity. Pretreatment of HPAEC with BiP siRNA inhibited thrombin-induced IκBα degradation and its resulting downstream signaling pathway involving NF-κB nuclear translocation, DNA binding, phosphorylation at serine536, transcriptional activation and subsequent expression of adhesion molecules. However, TNFα-mediated NF-κB signaling was unaffected upon BiP knockdown. In an alternative approach, SubAB-mediated inactivation of NF-κB was independent of IκBα degradation. Mechanistic analysis revealed that pretreatment of EC with SubAB interfered with the binding of the liberated NF-κB to the DNA, thereby resulting in reduced expression of adhesion molecules, cytokines and chemokines. In addition, both knockdown and inactivation of BiP stimulated actin cytoskeletal reorganization resulting in restoration of endothelial permeability. Together our studies indicate that BiP plays a central role in EC inflammation and injury via its action on NF-κB activation and regulation of vascular permeability.

Role of unfolded protein response dysregulation in oxidative injury of retinal pigment epithelial cells

AIMS

Age-related macular degeneration (AMD), a major cause of legal blindness in the elderly, is associated with genetic and environmental risk factors, such as cigarette smoking. Recent evidence shows that cigarette smoke (CS) that contains high levels of potent oxidants preferably targets retinal pigment epithelium (RPE) leading to oxidative damage and apoptosis; however, the mechanisms are poorly understood. The present study aimed to investigate the role of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in CS-related RPE apoptosis.

RESULTS

ER stress and proapoptotic gene C/EBP homologous protein (CHOP) were induced in the RPE/choroid complex from mice exposed to CS for 2 weeks and in human RPE cells treated with hydroquinone, a potent oxidant found at high concentrations in CS. Suppressing ER stress or inhibiting CHOP activation by pharmacological chaperones or genetic approaches attenuated hydroquinone-induced RPE cell apoptosis. In contrast to enhanced CHOP activation, protein level of active X-box binding protein 1 (XBP1), a major regulator of the adaptive UPR, was reduced in hydroquinone-treated cells. Conditional knockout of XBP1 gene in the RPE resulted in caspase-12 activation, increased CHOP expression, and decreased antiapoptotic gene Bcl-2. Furthermore, XBP1-deficient RPE cells are more sensitive to oxidative damage induced by hydroquinone or NaIO3, a CS-unrelated chemical oxidant. Conversely, overexpressing XBP1 protected RPE cells and attenuated oxidative stress-induced RPE apoptosis.

INNOVATION AND CONCLUSION

These findings provide strong evidence suggesting an important role of ER stress and the UPR in CS-related oxidative injury of RPE cells. Thus, the modulation of the UPR signaling may provide a promising target for the treatment of AMD.

Control of dopaminergic neuron survival by the unfolded protein response transcription factor XBP1

Parkinson disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although growing evidence indicates that endoplasmic reticulum (ER) stress is a hallmark of PD, its exact contribution to the disease process is not well understood. Here we report that developmental ablation of X-Box binding protein 1 (XBP1) in the nervous system, a key regulator of the unfolded protein response (UPR), protects dopaminergic neurons against a PD-inducing neurotoxin. This survival effect was associated with a preconditioning condition that resulted from induction of an adaptive ER stress response in dopaminergic neurons of the SNpc, but not in other brain regions. In contrast, silencing XBP1 in adult animals triggered chronic ER stress and dopaminergic neuron degeneration. Supporting this finding, gene therapy to deliver an active form of XBP1 provided neuroprotection and reduced striatal denervation in animals injected with 6-hydroxydopamine. Our results reveal a physiological role of the UPR in the maintenance of protein homeostasis in dopaminergic neurons that may help explain the differential neuronal vulnerability observed in PD.

Endoplasmic reticulum stress and the unfolded protein responses in retinal degeneration

The endoplasmic reticulum (ER) is the primary intracellular organelle responsible for protein and lipid biosynthesis, protein folding and trafficking, calcium homeostasis, and several other vital processes in cell physiology. Disturbance in ER function results in ER stress and subsequent activation of the unfolded protein response (UPR). The UPR up-regulates ER chaperones, reduces protein translation, and promotes clearance of cytotoxic misfolded proteins to restore ER homeostasis. If this vital process fails, the cell will be signaled to enter apoptosis, resulting in cell death. Sustained ER stress also can trigger an inflammatory response and exacerbate oxidative stress, both of which contribute synergistically to tissue damage. Studies performed over the past decade have implicated ER stress in a broad range of human diseases, including neurodegenerative diseases, cancer, diabetes, and vascular disorders. Several of these diseases also entail retinal dysfunction and degeneration caused by injury to retinal neurons and/or to the blood vessels that supply retinal cells with nutrients, trophic and homeostatic factors, oxygen, and other essential molecules, as well as serving as a conduit for removal of waste products and potentially toxic substances from the retina. Collectively, such injuries represent the leading cause of blindness world-wide in all age groups. Herein, we summarize recent progress on the study of ER stress and UPR signaling in retinal biology and discuss the molecular mechanisms and the potential clinical applications of targeting ER stress as a new therapeutic approach to prevent and treat neuronal degeneration in the retina.

Control of dopaminergic neuron survival by the unfolded protein response transcription factor XBP1

Parkinson disease (PD) is characterized by the selective loss of dopaminergic neurons of the substantia nigra pars compacta (SNpc). Although growing evidence indicates that endoplasmic reticulum (ER) stress is a hallmark of PD, its exact contribution to the disease process is not well understood. Here we report that developmental ablation of X-Box binding protein 1 (XBP1) in the nervous system, a key regulator of the unfolded protein response (UPR), protects dopaminergic neurons against a PD-inducing neurotoxin. This survival effect was associated with a preconditioning condition that resulted from induction of an adaptive ER stress response in dopaminergic neurons of the SNpc, but not in other brain regions. In contrast, silencing XBP1 in adult animals triggered chronic ER stress and dopaminergic neuron degeneration. Supporting this finding, gene therapy to deliver an active form of XBP1 provided neuroprotection and reduced striatal denervation in animals injected with 6-hydroxydopamine. Our results reveal a physiological role of the UPR in the maintenance of protein homeostasis in dopaminergic neurons that may help explain the differential neuronal vulnerability observed in PD.

The Caenorhabditis elegans HP1 family protein HPL-2 maintains ER homeostasis through the UPR and hormesis

Cellular adaptation to environmental changes and stress relies on a wide range of regulatory mechanisms that are tightly controlled at several levels, including transcription. Chromatin structure and chromatin binding proteins are important factors contributing to the transcriptional response to stress. However, it remains largely unknown to what extent specific chromatin factors influence the response to distinct forms of stress in a developmental context. One of the best characterized stress response pathways is the unfolded protein response (UPR), which is activated by accumulation of misfolded proteins in the endoplasmic reticulum (ER). Here, we show that Caenorhabditis elegans heterochromatin protein like-2 (HPL-2), the homolog of heterochromatin protein 1 (HP1), down-regulates the UPR in the intestine. Inactivation of HPL-2 results in an enhanced resistance to ER stress dependent on the X-box binding protein 1 (XBP-1)/inositol requiring enzyme 1 branch of the UPR and the closely related process of autophagy. Increased resistance to ER stress in animals lacking HPL-2 is associated with increased basal levels of XBP-1 activation and ER chaperone expression under physiological conditions, which may in turn activate an adaptive response known as ER hormesis. HPL-2 expression in intestinal cells is sufficient to rescue stress resistance, whereas expression in neuronal cells negatively influenced the ER stress response through a cell-nonautonomous mechanism. We further show that the retinoblastoma protein homolog LIN-35 and the LIN-13 zinc finger protein act in the same pathway as HPL-2 to limit the ER stress response. Altogether, our results point to multiple functions for HP1 in different cell types to maintain ER homeostasis.

ISMARA: automated modeling of genomic signals as a democracy of regulatory motifs

Accurate reconstruction of the regulatory networks that control gene expression is one of the key current challenges in molecular biology. Although gene expression and chromatin state dynamics are ultimately encoded by constellations of binding sites recognized by regulators such as transcriptions factors (TFs) and microRNAs (miRNAs), our understanding of this regulatory code and its context-dependent read-out remains very limited. Given that there are thousands of potential regulators in mammals, it is not practical to use direct experimentation to identify which of these play a key role for a particular system of interest. We developed a methodology that models gene expression or chromatin modifications in terms of genome-wide predictions of regulatory sites and completely automated it into a web-based tool called ISMARA (Integrated System for Motif Activity Response Analysis). Given only gene expression or chromatin state data across a set of samples as input, ISMARA identifies the key TFs and miRNAs driving expression/chromatin changes and makes detailed predictions regarding their regulatory roles. These include predicted activities of the regulators across the samples, their genome-wide targets, enriched gene categories among the targets, and direct interactions between the regulators. Applying ISMARA to data sets from well-studied systems, we show that it consistently identifies known key regulators ab initio. We also present a number of novel predictions including regulatory interactions in innate immunity, a master regulator of mucociliary differentiation, TFs consistently disregulated in cancer, and TFs that mediate specific chromatin modifications.

The unfolded protein response and diabetic retinopathy

Diabetic retinopathy, a common complication of diabetes, is the leading cause of blindness in adults. Diabetes chronically damages retinal blood vessels and neurons likely through multiple pathogenic pathways such as oxidative stress, inflammation, and endoplasmic reticulum (ER) stress. To relieve ER stress, the cell activates an adaptive mechanism known as the unfolded protein response (UPR). The UPR coordinates the processes of protein synthesis, protein folding, and degradation to ensure proteostasis, which is vital for cell survival and activity. Emerging evidence suggests that diabetes can activate all three UPR branches in retinal cells, among which the PERK/ATF4 pathway is the most extensively studied in the development of diabetic retinopathy. X-box binding protein 1 (XBP1) is a major transcription factor in the core UPR pathway and also regulates a variety of genes involved in cellular metabolism, redox state, autophagy, inflammation, cell survival, and vascular function. The exact function and implication of XBP1 in the pathogenesis of diabetic retinopathy remain elusive. Focusing on this less studied pathway, we summarize recent progress in studies of the UPR pertaining to diabetic changes in retinal vasculature and neurons, highlighting the perspective of XBP1 as a potential therapeutic target in diabetic retinopathy.

Bidirectional regulation of NF-κB by reactive oxygen species: a role of unfolded protein response

Nuclear factor-κB (NF-κB) is a transcription factor that plays a crucial role in coordinating innate and adaptive immunity, inflammation, and apoptotic cell death. NF-κB is activated by various inflammatory stimuli including peptide factors and infectious microbes. It is also known as a redox-sensitive transcription factor activated by reactive oxygen species (ROS). Over the past decades, various investigators focused on the role of ROS in the activation of NF-κB by cytokines and lipopolysaccharides. However, recent studies also suggested that ROS have the potential to repress NF-κB activity. Currently, it is not well addressed how ROS regulate activity of NF-κB in a bidirectional fashion. In this paper, we summarize evidence for positive and negative regulation of NF-κB by ROS, possible redox-sensitive targets for NF-κB signaling, and mechanisms underlying biphasic and bidirectional influences of ROS on NF-κB, especially focusing on a role of ROS-mediated induction of endoplasmic reticulum stress.

Multi-targeted mechanisms underlying the endothelial protective effects of the diabetic-safe sweetener erythritol

Diabetes is characterized by hyperglycemia and development of vascular pathology. Endothelial cell dysfunction is a starting point for pathogenesis of vascular complications in diabetes. We previously showed the polyol erythritol to be a hydroxyl radical scavenger preventing endothelial cell dysfunction onset in diabetic rats. To unravel mechanisms, other than scavenging of radicals, by which erythritol mediates this protective effect, we evaluated effects of erythritol in endothelial cells exposed to normal (7 mM) and high glucose (30 mM) or diabetic stressors (e.g. SIN-1) using targeted and transcriptomic approaches. This study demonstrates that erythritol (i.e. under non-diabetic conditions) has minimal effects on endothelial cells. However, under hyperglycemic conditions erythritol protected endothelial cells against cell death induced by diabetic stressors (i.e. high glucose and peroxynitrite). Also a number of harmful effects caused by high glucose, e.g. increased nitric oxide release, are reversed. Additionally, total transcriptome analysis indicated that biological processes which are differentially regulated due to high glucose are corrected by erythritol. We conclude that erythritol protects endothelial cells during high glucose conditions via effects on multiple targets. Overall, these data indicate a therapeutically important endothelial protective effect of erythritol under hyperglycemic conditions.

Vascular endothelial cell growth-activated XBP1 splicing in endothelial cells is crucial for angiogenesis

BACKGROUND

Vascular endothelial cell growth factor plays a pivotal role in angiogenesis via regulating endothelial cell proliferation. The X-box binding protein 1 (XBP1) is believed to be a signal transducer in the endoplasmic reticulum stress response. It is unknown whether there is crosstalk between vascular endothelial cell growth factor signaling and XBP1 pathway.

METHODS AND RESULTS

We found that vascular endothelial cell growth factor induced the kinase insert domain receptor internalization and interaction through C-terminal domain with the unspliced XBP1 and the inositol requiring enzyme 1 α in the endoplasmic reticulum, leading to inositol requiring enzyme 1 α phosphorylation and XBP1 mRNA splicing, which was abolished by siRNA-mediated knockdown of kinase insert domain receptor. Spliced XBP1 regulated endothelial cell proliferation in a PI3K/Akt/GSK3β/β-catenin/E2F2-dependent manner and modulated the cell size increase in a PI3K/Akt/GSK3β/β-catenin/E2F2-independent manner. Knockdown of XBP1 or inositol requiring enzyme 1 α decreased endothelial cell proliferation via suppression of Akt/GSK3β phosphorylation, β-catenin nuclear translocation, and E2F2 expression. Endothelial cell-specific knockout of XBP1 (XBP1ecko) in mice retarded the retinal vasculogenesis in the first 2 postnatal weeks and impaired the angiogenesis triggered by ischemia. Reconstitution of XBP1 by Ad-XBP1s gene transfer significantly improved angiogenesis in ischemic tissue in XBP1ecko mice. Transplantation of bone marrow from wild-type o XBP1ecko mice could also slightly improve the foot blood reperfusion in ischemic XBP1ecko mice.

CONCLUSIONS

These results suggest that XBP1 can function via growth factor signaling pathways to regulate endothelial proliferation and angiogenesis.

A mathematical model of the unfolded protein stress response reveals the decision mechanism for recovery, adaptation and apoptosis

Background

The unfolded protein response (UPR) is a major signalling cascade acting in the quality control of protein folding in the endoplasmic reticulum (ER). The cascade is known to play an accessory role in a range of genetic and environmental disorders including neurodegenerative and cardiovascular diseases, diabetes and kidney diseases. The three major receptors of the ER stress involved with the UPR, i.e. IRE1 α, PERK and ATF6, signal through a complex web of pathways to convey an appropriate response. The emerging behaviour ranges from adaptive to maladaptive depending on the severity of unfolded protein accumulation in the ER; however, the decision mechanism for the switch and its timing have so far been poorly understood.

Results

Here, we propose a mechanism by which the UPR outcome switches between survival and death. We compose a mathematical model integrating the three signalling branches, and perform a comprehensive bifurcation analysis to investigate possible responses to stimuli. The analysis reveals three distinct states of behaviour, low, high and intermediate activity, associated with stress adaptation, tolerance, and the initiation of apoptosis. The decision to adapt or destruct can, therefore, be understood as a dynamic process where the balance between the stress and the folding capacity of the ER plays a pivotal role in managing the delivery of the most appropriate response. The model demonstrates for the first time that the UPR is capable of generating oscillations in translation attenuation and the apoptotic signals, and this is supplemented with a Bayesian sensitivity analysis identifying a set of parameters controlling this behaviour.

Conclusions

This work contributes largely to the understanding of one of the most ubiquitous signalling pathways involved in protein folding quality control in the metazoan ER. The insights gained have direct consequences on the management of many UPR-related diseases, revealing, in addition, an extended list of candidate disease modifiers. Demonstration of stress adaptation sheds light to how preconditioning might be beneficial in manifesting the UPR outcome to prevent untimely apoptosis, and paves the way to novel approaches for the treatment of many UPR-related conditions.

Signalling danger: endoplasmic reticulum stress and the unfolded protein response in pancreatic islet inflammation

Protein synthesis is increased by several-fold in stimulated pancreatic beta cells. Synthesis and folding of (pro)insulin takes place in the endoplasmic reticulum (ER), and beta cells trigger the unfolded protein response (UPR) to upgrade the functional capacity of the ER. Prolonged or excessive UPR activation contributes to beta cell dysfunction and death in type 2 diabetes, but there is another side of the UPR that may be of particular relevance for autoimmune type 1 diabetes, namely, the cross-talk between the UPR and innate immunity/inflammation. Recent evidence, discussed in this review, indicates that both saturated fats and inflammatory mediators such as cytokines trigger the UPR in pancreatic beta cells. The UPR potentiates activation of nuclear factor κB, a key regulator of inflammation. Two branches of the UPR, namely IRE1/XBP1s and PERK/ATF4/CHOP, mediate the UPR-induced sensitisation of pancreatic beta cells to the proinflammatory effects of cytokines. This can contribute to the upregulation of local inflammatory mechanisms and the aggravation of insulitis. The dialogue between the UPR and inflammation may provide an explanation for the parallel increase in the prevalence of childhood obesity and type 1 diabetes.

Quinotrierixin inhibits proliferation of human retinal pigment epithelial cells

PURPOSE

To investigate the effect of quinotrierixin, a previously reported inhibitor of X-box binding protein 1 (XBP1), on cell proliferation and viability in human retinal pigment epithelium (RPE) cells.

METHODS

Subconfluent human RPE cells (ARPE-19) were exposed to quinotrierixin for 16-24 h. Cell proliferation was determined with 3-(4, 5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide assay, hemocytometer counts, and CyQUANT NF Cell Proliferation Assay. Apoptosis was detected with terminal deoxynucleotidyl transferase-mediated uridine 5'-triphosphate-biotin nick end labeling assay. XBP1 mRNA splicing and expression of endoplasmic reticulum stress response genes were determined in cells exposed to thapsigargin in the presence or absence of quinotrierixin. Overexpression of spliced XBP1 was achieved with adenovirus.

RESULTS

Quinotrierixin reduced RPE cell proliferation in a dose-dependent manner without inducing apoptosis. In cells exposed to thapsigargin, quinotrierixin inhibited XBP1 mRNA splicing and PKR-like endoplasmic reticulum kinase activation, and reduced cellular and nuclear levels of spliced XBP1 and C/EBP homologous protein. Paradoxically, quinotrierixin exacerbated endoplasmic reticulum stress-induced phosphorylation of eIF2α, which in turn led to decreased protein translation. Overexpressing spliced XBP1 partially reversed the inhibition of cell proliferation by quinotrierixin. These results suggest that inhibiting XBP1 splicing contributes to quinotrierixin's negative effect on RPE cell proliferation, but other mechanisms such as reduction of protein translation are also involved.

CONCLUSIONS

Quinotrierixin inhibits RPE cell proliferation and may be used as a novel antiproliferative drug for treating proliferative vitreoretinopathy. Future studies are needed to investigate the in vivo effect of quinotrierixin on RPE proliferation in animal models of proliferative vitreoretinopathy.

Activating transcription factor 4 mediates hyperglycaemia-induced endothelial inflammation and retinal vascular leakage through activation of STAT3 in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

There is convincing evidence that endoplasmic reticulum (ER) stress is implicated in the pathogenesis of diabetes and its complications; however, the mechanisms are not fully understood. This study aimed to dissect the role and signalling pathways of activating transcription factor 4 (ATF4) in ER-stress-associated endothelial inflammation and diabetic retinopathy.

METHODS

ER stress and ATF4 activity were manipulated by complementary pharmacological and genetic approaches in cultured retinal endothelial (TR-iBRB) cells. Diabetes was induced by streptozotocin in heterozygous Atf4 knockout and wild-type mice. ER stress markers, inflammatory cytokines and adhesion molecules, activation of the signal transducer and activator of transcription 3 (STAT3) pathway, and retinal vascular permeability were measured.

RESULTS

High-glucose treatment resulted in rapid induction of ER stress, activation of ATF4, and increased production of inflammatory factors in TR-iBRB cells. Suppressing ER stress or inhibiting ATF4 activity markedly attenuated high-glucose-induced production of intercellular adhesion molecule 1, TNF-α and vascular endothelial growth factor. Conversely, enhancing ER stress or overexpressing Atf4 was sufficient to induce endothelial inflammation, which was, at least in part, through activation of the STAT3 pathway. Furthermore, knockdown of the Stat3 gene or inhibiting STAT3 activity restored ER homeostasis in cells exposed to high glucose and prevented ATF4 activation, suggesting that STAT3 is required for high-glucose-induced ER stress. Finally, we showed that downregulation of Atf4 significantly ameliorated retinal inflammation, STAT3 activation and vascular leakage in a mouse model of type 1 diabetes.

CONCLUSIONS/INTERPRETATION

Taken together, our data reveal a pivotal role of ER stress and the ATF4/STAT3 pathway in retinal endothelial inflammation in diabetic retinopathy.

Endoplasmic reticulum stress modulates liver inflammatory immune response in the pathogenesis of liver ischemia and reperfusion injury

BACKGROUND

Although endoplasmic reticulum (ER) stress has been implicated in the pathophysiology of organ ischemia-reperfusion injury (IRI), the underlying mechanisms have yet to be fully elucidated. In particular, because tissue proinflammatory immune response is the key mediator of local IRI, how ER stress impacts liver immune cell activation cascade remains to be determined.

METHODS

In vitro, ER stress in macrophages and hepatocytes were induced by pharmacological agents. Macrophage Toll-like receptor 4 and hepatocyte tumor necrosis factor α responses were studied. In vivo, the induction of ER stress by ischemia-reperfusion and the impact of ER stress amelioration by a small molecule chaperon 4-phenylbutyric acid on liver immune response were studied in a murine partial liver warm ischemia model.

RESULTS

ER-stressed macrophages generated a significantly enhanced proinflammatory immune response against Toll-like receptor 4 stimulation, whereas ER-stressed hepatocytes became more susceptible to tumor necrosis factor α-induced cell death. Ischemia-reperfusion resulted in up-regulations of spliced X-box binding protein 1 and activating transcription factor 6 levels in affected livers. Mice pretreated with 4-phenylbutyric acid were protected from liver IRI, in parallel with diminished local proinflammatory gene induction program.

CONCLUSIONS

Our study documents a potential immune regulatory role of ER stress in the mechanism of liver IRI and provides a rationale for targeting stress response as a new therapeutic means to ameliorate tissue inflammation in organ transplant recipients.

X-box binding protein 1 is essential for the anti-oxidant defense and cell survival in the retinal pigment epithelium

Damage to the retinal pigment epithelium (RPE) is an early event in the pathogenesis of age-related macular degeneration (AMD). X-box binding protein 1 (XBP1) is a key transcription factor that regulates endoplasmic reticulum (ER) homeostasis and cell survival. This study aimed to delineate the role of endogenous XBP1 in the RPE. Our results show that in a rat model of light-induced retinal degeneration, XBP1 activation was suppressed in the RPE/choroid complex, accompanied by decreased anti-oxidant genes and increased oxidative stress. Knockdown of XBP1 by siRNA resulted in reduced expression of SOD1, SOD2, catalase, and glutathione synthase and sensitized RPE cells to oxidative damage. Using Cre/LoxP system, we generated a mouse line that lacks XBP1 only in RPE cells. Compared to wildtype littermates, RPE-XBP1 KO mice expressed less SOD1, SOD2, and catalase in the RPE, and had increased oxidative stress. At age 3 months and older, these mice exhibited apoptosis of RPE cells, decreased number of cone photoreceptors, shortened photoreceptor outer segment, reduced ONL thickness, and deficit in retinal function. Electron microscopy showed abnormal ultrastructure, Bruch's membrane thickening, and disrupted basal membrane infolding in XBP1-deficient RPE. These results indicate that XBP1 is an important gene involved in regulation of the anti-oxidant defense in the RPE, and that impaired activation of XBP1 may contribute to RPE dysfunction and cell death during retinal degeneration and AMD.

Hormesis: protecting neurons against cellular stress in Parkinson disease

Protein folding stress is a salient feature of the most frequent neurodegenerative diseases. Although the accumulation of abnormally folded proteins is a well-characterized event underlying the pathology, the way cells respond to this phenomenon is not well understood. Signs of endoplasmic reticulum (ER) stress are a common marker of neurodegeneration in many diseases, which may represent two contrasting processes: cell protection events due to activation of adaptive programs, or a chronic stress state that culminates in apoptosis to eliminate irreversibly injured cells. Autophagy has been proposed as a protective mechanism to overcome neurodegeneration that is also modulated by ER stress. In this issue of autophagy Bertrand Mollereau's group provides novel evidence indicating that engagement of nonharmful levels of ER stress protects against experimental Parkinson disease. At the mechanistic level, a homeostatic crosstalk between ER stress signaling and the autophagy pathway was proposed to mediate the therapeutic effects. This study, together with recent findings, supports the involvement of a "hormesis mechanism" to handle degeneration through preconditioning mediated by a dynamic balance between ER stress and autophagy. The implications for aging and future therapeutic development are discussed.

Inflammation, endoplasmic reticulum stress, autophagy, and the monocyte chemoattractant protein-1/CCR2 pathway

Numerous inflammatory cytokines have been implicated in the pathogenesis of cardiovascular diseases. Monocyte chemoattractant protein (MCP)-1/CCL2 is expressed by mainly inflammatory cells and stromal cells such as endothelial cells, and its expression is upregulated after proinflammatory stimuli and tissue injury. MCP-1 can function as a traditional chemotactic cytokine and also regulates gene transcription. The recently discovered novel zinc-finger protein, called MCPIP (MCP-1-induced protein), initiates a series of signaling events that causes oxidative and endoplasmic reticulum (ER) stress, leading to autophagy that can result in cell death or differentiation, depending on the cellular context. After a brief review of the basic processes involved in inflammation, ER stress, and autophagy, the recently elucidated role of MCP-1 and MCPIP in inflammatory diseases is reviewed. MCPIP was found to be able to control inflammatory response by inhibition of nuclear factor-κB activation through its deubiquitinase activity or by degradation of mRNA encoding a set of inflammatory cytokines through its RNase activity. The potential inclusion of such a novel deubiquitinase in the emerging anti-inflammatory strategies for the treatment of inflammation-related diseases such as cardiovascular diseases and type 2 diabetes is briefly discussed.

TXNIP links innate host defense mechanisms to oxidative stress and inflammation in retinal Muller glia under chronic hyperglycemia: implications for diabetic retinopathy

Thioredoxin Interacting Protein (TXNIP) mediates retinal inflammation, gliosis, and apoptosis in experimental diabetes. Here, we investigate the temporal response of Muller glia to high glucose (HG) and TXNIP expression using a rat Muller cell line (rMC1) in culture. We examined if HG-induced TXNIP expression evokes host defense mechanisms in rMC1 in response to metabolic abnormalities. HG causes sustained up-regulation of TXNIP (2 h to 5 days), ROS generation, ATP depletion, ER stress, and inflammation. Various cellular defense mechanisms are activated by HG: (i) NLRP3 inflammasome, (ii) ER stress response (sXBP1), (iii) hypoxic-like HIF-1α induction, (iv) autophagy/mitophagy, and (v) apoptosis. We also found in vivo that streptozocin-induced diabetic rats have higher retinal TXNIP and innate immune response gene expression than normal rats. Knock down of TXNIP by intravitreal siRNA reduces inflammation (IL-1β) and gliosis (GFAP) in the diabetic retina. TXNIP ablation in vitro prevents ROS generation, restores ATP level and autophagic LC3B induction in rMC1. Thus, our results show that HG sustains TXNIP up-regulation in Muller glia and evokes a program of cellular defense/survival mechanisms that ultimately lead to oxidative stress, ER stress/inflammation, autophagy and apoptosis. TXNIP is a potential target to ameliorate blinding ocular complications of diabetic retinopathy.

Activation of endoplasmic reticulum stress by hyperglycemia is essential for Müller cell-derived inflammatory cytokine production in diabetes

Inflammation plays an important role in diabetes-induced retinal vascular leakage. The purpose of this study is to examine the role of endoplasmic reticulum (ER) stress and the signaling pathway of ER stress-induced activating transcription factor 4 (ATF4) in the regulation of Müller cell-derived inflammatory mediators in diabetic retinopathy. In diabetic animals, elevated ER stress markers, ATF4, and vascular endothelial growth factor (VEGF) expression were partially localized to Müller cells in the retina. In cultured Müller cells, high glucose induced a time-dependent increase of ER stress, ATF4 expression, and inflammatory factor production. Inducing ER stress or overexpressing ATF4 resulted in elevated intracellular adhesion molecule 1 and VEGF proteins in Müller cells. In contrast, alleviation of ER stress or blockade of ATF4 activity attenuated inflammatory gene expression induced by high glucose or hypoxia. Furthermore, we found that ATF4 regulated the c-Jun NH2-terminal kinase pathway resulting in VEGF upregulation. ATF4 was also required for ER stress-induced and hypoxia-inducible factor-1α activation. Finally, we showed that administration of chemical chaperone 4-phenylbutyrate or genetic inhibition of ATF4 successfully attenuated retinal VEGF expression and reduced vascular leakage in mice with STZ-induced diabetes. Taken together, our data indicate that ER stress and ATF4 play a critical role in retinal inflammatory signaling and Müller cell-derived inflammatory cytokine production in diabetes.

Endoplasmic reticulum stress and inflammation: mechanisms and implications in diabetic retinopathy

The endoplasmic reticulum (ER) is the primary cellular compartment where proteins are synthesized and modified before they can be transported to their destination. Dysfunction of the ER impairs protein homeostasis and leads to the accumulation of misfolded/unfolded proteins in the ER, or ER stress. While it has long been recognized that ER stress is a major cause of conformational disorders, such as Alzheimer's disease, Huntington's disease, certain types of cancer, and type 2 diabetes, recent evidence suggests that ER stress is also implicated in many chronic inflammatory diseases. These diseases include irritable bowel syndrome, atherosclerosis, diabetic complications, and many others. Diabetic retinopathy is a common microvascular complication of diabetes, characterized by chronic inflammation, progressive damage to retinal vascular and neuronal cells, vascular leakage, and abnormal blood vessel growth (neovascularization). In this review, we discuss the role and mechanisms of ER stress in retinal inflammation and vascular damage in diabetic retinopathy.

The interplay between endoplasmic reticulum stress and inflammation

Endoplasmic reticulum (ER) stress may be both a trigger and consequence of chronic inflammation. Chronic inflammation is often associated with diseases that arise because of primary misfolding mutations and ER stress. Similarly, ER stress and activation of the unfolded protein response (UPR) is a feature of many chronic inflammatory and autoimmune diseases. In this review, we describe how protein misfolding and the UPR trigger inflammation, how environmental ER stressors affect antigen presenting cells and immune effector cells, and present evidence that inflammatory factors exacerbate protein misfolding and ER stress. Examples from both animal models of disease and human diseases are used to illustrate the complex interactions between ER stress and inflammation, and opportunities for therapeutic targeting are discussed. Finally, recommendations are made for future research with respect to the interaction of ER stress and inflammation.

Autoimmunity occurs when an organism develops an immune response against itself, resulting in an inflammatory reaction which damages organs such as brain, joints or pancreas. This results in diseases such as Type 1 diabetes, vasculitis, or rheumatoid arthritis. A fine balance exists in order to accommodate the control of microbial pathogens and commensals, and immune self‐tolerance. The March 2012 issue will include a review series on Autoimmune Disease, particularly featuring articles on clinical translation, and the current state of research in this area. Articles include reasons for the increased incidence of certain autoimmune diseases and allergic diseases in Western society and the advances made by the application of novel and high throughput technologies to the analysis of diseased tissues. The accompanying web focus presents links to related articles from across Nature Publishing Group.

ER stress and apoptosis: a new mechanism for retinal cell death

The endoplasmic reticulum (ER) is the primary subcellular organelle where proteins are synthesized and folded. When the homeostasis of the ER is disturbed, unfolded or misfolded proteins accumulate in the ER lumen, resulting in ER stress. In response to ER stress, cells activate a set of tightly controlled regulatory programs, known as the unfolded protein response (UPR), to restore the normal function of the ER. However, if ER stress is sustained and the adaptive UPR fails to eliminate unfolded/misfolded proteins, apoptosis will occur to remove the stressed cells. In recent years, a large body of studies has shown that ER stress-induced apoptosis is implicated in numerous human diseases, such as diabetes and neurogenerative diseases. Moreover, emerging evidence supports a role of ER stress in retinal apoptosis and cell death in blinding disorders such as age-related macular degeneration and diabetic retinopathy. In the present review, we summarize recent progress on ER stress and apoptosis in retinal diseases, focusing on various proapoptotic and antiapoptotic pathways that are activated by the UPR, and discuss how these pathways contribute to ER stress-induced apoptosis in retinal cells.

Endoplasmic reticulum stress-related factors protect against diabetic retinopathy

The endoplasmic reticulum (ER) is a principal mediator of signal transduction in the cell, and disruption of its normal function (a mechanism known as ER stress) has been associated with the pathogenesis of several diseases. ER stress has been demonstrated to contribute to onset and progression of diabetic retinopathy (DR) by induction of multiple inflammatory signaling pathways. Recent studies have begun to describe the gene expression profile of ER stress-related genes in DR; moreover, genes that play a protective role against DR have been identified. P58^IPK was determined to be able to reduce retinal vascular leakage under high glucose conditions, thus protecting retinal cells. It has also been found by our lab that ER-associated protein degradation factors exhibit significantly different expression patterns in rat retinas under sustained high glucose conditions. Future research based upon these collective genomic findings will contribute to our overall understanding of DR pathogenesis as well as identify potential therapeutic targets.

Resistance to type 2 diabetes mellitus: a matter of hormesis?

Type 2 diabetes mellitus is characterized by subclinical systemic inflammation and impaired regulation of blood glucose levels. Interestingly, impairment of glycemic control occurs despite substantial insulin secretion early in the course of this disease. Dysfunction of several organs (including pancreatic islets, liver, skeletal muscle, adipose tissue, gut, hypothalamus and the immune system) has been implicated in the pathogenesis of type 2 diabetes mellitus. However, diabetes-promoting lifestyle factors do not inevitably cause disease in all persons exposed. Hence, defense mechanisms must exist that can keep the detrimental influence of these risk factors at bay. Hormesis describes the phenomenon that exposure to a mild stressor confers resistance to subsequent, otherwise harmful, conditions of increased stress. This Review discusses the emerging concept that the effectiveness of an adaptive (hormetic) response to detrimental lifestyle factors determines the extent of protection from progression to type 2 diabetes mellitus. Further analysis of these protective hormetic responses at the molecular level should help to identify novel targets for preventive or therapeutic intervention in patients at risk of developing type 2 diabetes mellitus or those with overt disease.